Synchronous selective neutralizing system



Aug- 4, 1942 J. w. MILNOR r-:TAL 2,291,825

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5 Sheets-Sheet 2 A Z ber ,230396 :Attozuag/ Aug. 4 1942'. J- W- MII-NOR ETAI- SYNCHRONOUS SELECTIVE NEUTRALIZING SYSTEM Filed DecA 29, 1959 5 Sheets-Sheet 3 Sync/#1m ou s ,A-ber oigo J. W. MILNOR ETAL SYNCHRONOUS SELECTIVE NEUTRALIZING SYSTEM Aug. 4, 1942.

Filed Dec. 29, 19259 5 Sheets-Sheet 4 J WMzlZnor WD. C'a/mwn @W/fm y Aug. 4, 1942. J W- MLNOR HAL 2,291,825

SYNCHRONOUS SELECTIVE NEUTRALIZING SYSTEM Filed Deo. 29, 1939 5 sheets-sheet 5 Patented Aug. 4, `1942 unirse STATES vT OFFICE SYNCHRONOUS SELECTIVE NEUTRALIZING SYSTEM Application December 29, 1939, Serial No. 311,686

17 Claims.

This invention relates to a system for the neutralization of interfering potentials caused by extraneous induction that may be produced in telegraph or other communication circuits by inductive interference from adjacent power, lighting or railway circuits or by any other cause extraneous to the communication circuits.

Various systems for neutralization power circuit frequency interference induced in telegraph or other communication systems have been proposed and some of them have been found to be highly practical. In these prior systems, a control wire paralleling the communication circuits is employed which is subject to the disturbing inductive interference from power circuits, or other extraneous source and serves to gather interferencepotentialsoflike frequency and magnitude that may be used to control the neutralizing system. In some systems the control wire must be of a special construction and in any case, is considerably degraded for communication purposes.

One of the objects of our invention is to eliminate the customary control wire and to provide a system in which the neutralizing potential is obtained from a local source independent of the communication circuits and is maintained in proper neutralizing relation as to phase and magnitude through the functioning of the neutralizing system itself. The control potential for maintaining this relation is selected from the communication wires themselves, whence it acts upon the frequency control mechanism via a circuit of high time constant to produce an eiiect analogous to that which would be obtained if extremely sharp tuning were employed in selecting the control frequency. The neutralizing potential source must be independent of the line wires.

Another object of our invention is to minimize and to practically prevent the neutralizing system from functioning at the frequency of the telegraph signals and thereby eliminate any tendency to remove useful components of the signals.

Other objects will be apparent from the following description of the annexed drawings in which:

Figure 1 represents diagrammatically an elemental form of our neutralizing system applied to a group of communication line wires that may be exposed to inductive interference by reason of their proximity to power circuits or other extraneous source, in which the customary control of the system is eliminated and the controlling potential is obtained through a common coupling to the line wires.

Figure 2 illustrates another arrangement embodying our neutralizing system applied to a group of communication line wires exposed to inductive interference by reason of their proximity to power circuits or vother extraneous source, wherein the neutralizing frequency is derived from a source separate from the line wires of the communication circuits.

Figure 2a is a vector diagram referred to in following vdescription of the manner of obtaining the proper neutralizing voltage.

Figures 2b and 2c illustrate graphically the manner in which tubes A and A function to produce a D. C. potential Varying in value as the applied disturbing potential varies.

Figure 2d illustrates the manner of producing a neutralizing voltage of any desired phase relation with respect to the source I6.

Figure 3 shows a modification similar to that of Fig. 2, wherein a saving of apparatus is effected by arranging the devices connected to the transformers in the control circuit to provide complete phase reversals.

Figure 4 illustrates further modification of our neutralizing system showing another method of providing a 360 rotation of the neutralizing voltage.

Figure 4a illustrates the relative positions of the potentials of the disturbing current and the current from the source I6 in the arrangement of Figure 4, as applied to the rectifier tubes under different conditions of phase difference.

Figure 5 illustrates an arrangement of the power interference neutralizing system disclosed herein applied to the neutralizing transformer scheme of Patent 2,159,927 by utilizing a mechanical rectifier and generator of the type shown in Patent 1,688,735.

We have illustrated in Figure 1, a group of communication line wires that may be exposed to inductive interference from adjacent power circuits or other extraneous source and showing an elemental form of our neutralizing system employing a neutralizing transformer arrangement of the type shown in Fig. 2 of Patent No. 2,159,927, and wherein the necessary control potential is secured through a common coupling to the line wires by means of the transformer l. Tuning circuits are provided for the purpose of excluding all except the interfering power frequency and since only a residual voltage is present in the line wires, a high degree of amplification must be provided between the pick up transformer I and the neutralizing or compensation transformer 2. The selectivity of the tuning circuits must be of very high order in order to prevent the system from functioning also at the frequency of the telegraph signals present on the lines. This tendency to remove useful components of telegraph signals is minimized somewhat by the condition that the pick up transformer I is affected by the average current in all of the line wires. It may be possible also to select for control purposes, wires carrying relatively slow speed circuits. Instead of the electrical selective circuits indicated, mechanically selective circuits such as tuning forks tuned to the power frequency may be employed.

While the system of Fig. 1 will operate satisfactorily under some conditions, it bears the objection that with the best selective systems available it is not possible to entirely exclude useful signal components and as a consequence there is a certain amount of neutralization of these frequencies with the resultant impairment of the telegraph signals.

The neutralizing frequency may be obtained from a source separate from the line Wires, its magnitude and phase being controlled continuously in accordance with the instantaneous values of the disturbing voltages present in the line wires. For the external source of voltage a connection to the disturbing power source itself is preferred. In some cases it may be necessary to provide an auxiliary source as a supply of neutralizing potential.

Figure 2 illustrates schematically one system of this latter type At I0 is shown a neutralizing or compensating transformer which may be of any of the types shown in Patent 2,159,927. A feature of this invention is that the control voltage for this transformer is supplied from the system shown generally at Il, thus dispensing with the control wire required in the system of the patent. The arrangement for securing the control potential comprises a pick up connection I2 to each of the line wires, a selective circuit I3 and four separate amplifying and phase control systems I4, I5, 25 and 26. Referring particularly to the device I4, it is seen that the first stage comprises a pair of thermionic tubes A and A with their input circuits coupled in balanced relation to the control potential. To the common portion of this input circuit there is supplied a potential from a local alternator I operating synchronously with the disturbing source. This control system comprises a conventional suppressed carrier type demodulator with like frequencies applied to both the signal and carrier input circuits. When these two potentials as applied to tube A are alike in phase, the output potential of the pair of demodulator tubes across the resistance Il will have a maximum positive value and the tube IB will amplify with prescribed gain, a potential from the alternator I6 applied through a transformer I9. The output potential from this final tube I8, is then transmitted via transformer 23 to the neutralizing transformer system where it functions to control the degree of neutralization. The device Ill serves to supply a potential bearing the phase condition of generator i6 but varying in magnitude as the phase of the disturbing potential from the line wires varies from +90 to 90 with respect to the source IB, the maximum output occurring at zero phase. difference.

This action of the pair of demodulator tubes is somewhat analogous to that which occurs in the demodulation of signal voltages, but differs in important practical respects in that the frequencies are quite low in value and a D. C. component is desired in the output. By reference to Figs. 2b and 2c the exact manner in which tubes A and A function to produce a D. C. potential ranging in value between zero and a maximum positive value as the applied disturbing potential varies over a i degree phase range will be apparent. Figure 2b represents in the curves A and A' the grid potential-plate current characteristics of the two tubes A and A operating in opposed relation. The tubes are biased by means of the grid battery to operate in the curved part of the characteristics indicated by the intersection of the line 2 2 with the curves A and A to produce the steady state currents corresponding to lines y and y. The local potential from .the generator I6, represented by the dotted line curve b, excites the grids of the two tubes alike. The incoming disturbing potential as applied to the two grids is represented by the solid curve a, so that the two grids are excited in opposition. The gure illustrates the case in which the two potentials applied to the grid of tube A are in phase to produce the summation curve c, while the two potentials as applied to tube A are displaced to produce a zero summation voltage. The grid potential of tube A will produce in the plate circuit of that tube a current represented by curve d, in which the positive half cycles predominate over the negative half cycles to produce a considerable D. C. component. The output of tube A', however, remains unchanged in view of the absence of an exciting grid potential. t is now apparent that when the local potential and the incoming disturbing potential are in phase at tube A, a maximum positive potential will be applied to the condenser 2 I. It is clear also that if the phase of the incoming potential is changed by the above condition will be reversed to produce a maximum negative potential across the condenser 2|.

Consider now the case where the two applied potentials differ in phase by 90. Referring to Fig. 2c, curve b represents the local potential from source I6 as before, while curve a represents the incoming disturbing potential and is displaced in phase from the local potential by +90 as regards tube A and by 90 (or 270) as regards tube A. These two potentials add in the respective grid circuits of each tube to produce the curves c of like magnitude but displaced 90 with respect to each other. The resultant plate currents represented by the two curves d are then alike both in magnitude and phase to produce a net zero potential across the condenser 2|.

The network comprising the resistance 20 and the condenser 2l, comprise a high time constant circuit whose integrating action smooths out the rectied pulses appearing across the resistance II to provide a substantially constant potential for application to the amplifying tube I8. The tube I8 therefore amplies the power frequency from generator I6 in accordance with the regulating potential developed by tubes A, A. Amplier IB is normally biased heavily negative, so as to be operative only through the positive range of control potential. This positive range extends from the maximum, corresponding to the condition outlined in Fig. 2b, where the disturbing potential and the local potential are in phase as applied to tube A, to the condition represented by Figure 2c where these two potentials are i90 displaced. As the displacement continues, the grid of tube I8 is only rendered further negative and the output remains zero.

The selection of the disturbing potential for control purposes is limited to a`relatively narrow range of frequencies by the tuned circuit I3. The selectivity of this circuit, however, is inadequate to prevent some reception of the telegraph signal components whose frequency may adjoin the disturbing frequency. This shortcoming, however, is completely compensated by the effect of the resistance 20 and the condenser 2|. By adjusting these elements to provide a high time constant for the control system, the neutralizing system is not susceptible to frequencies even slightly displaced from the control frequency and is thus rendered independent of momentary control potentials differing even slightly in frequency from the power frequency, By this important addition, immunity from signal voltages is obtained. This degree of immunity is controllable by adjustment of the resistance-condenser network 20--2I, the setting adopted being a compromise between immunity to signal voltages and response to frequency and amplitude changes in the disturbing frequency. In one case a resistance of l megohm was used in combination with a condenser of 60 microfarads to produce a time constant of three seconds.

The single demodulator-amplifier combination above described suffices to neutralize any disturbance voltage which is in exact phase with the potential that appears at the transformer I9. However, since the local source and the disturbing frequency, while synchronized will not normally remain continuously in exact phase, it is necessary to provide a means of phase variation for the local source. In this figure a method is outlined which provides a 360 phase rotation for the ultimate neutralizing voltage. Referring to Fig. 2a let the reference vector o-a, represent the phase direction of the voltage appearing at transformer 23. The vector o-b, 90 displaced, represents the potential appearing at transformer 24 By regulating the potentials corresponding to the vectors o-a and o-b it is possible to produce a neutralizing voltage having any desired magnitude and phase but limited to the quadrant a-o-b; for example, the voltage corresponding to vector o-e. A further derivative of this vector will be given presently. By providing a second pair of potential sources operating into transformers 2'I and 28 and having phase relations represented by vectors o-c and o-cl it is evident that a neutralizing voltage may be introduced into the circuit 22 having all possible degrees of magnitude and phase.

The voltage corresponding to vector o--by is obtained by applying to the second demodulator I5 a demodulating voltage from the generator I6, but displaced in phase therefrom by 90 by the phase shifting device` 29. The remainder of this demodulator-amplifier combination is identical with the one first described and functions in the same manner to produce in the transformer 24 a regulated voltage displaced in phase 90 from that appearing at the transformer 23. To obtain the voltages corresponding to the vectors o-c and o d, an additional pair of demodulatorampliers is provided identical with the first pair, except that the demodulating frequency is displaced 180 so that there is produced in the transformers 2'I and 28 voltages displaced 180 and 270 degrees respectively with respect to the reference vector o-a.

In this figure the pick up connection to the line wires is via a group of high impedance taps. A transformer as shown in Fig. 1, or other similar connection, however, may be used instead. Also, while selective circuits have been shown in association with the pick-ups, their influence is small as compared with the high time constant of the system. The primary function of any tuning at this point is to correct phase shifts caused by other elements of the system and `in some cases may be omitted.

The production of a neutralizing voltage of any desired phase relation with respect to source I6 is further illustrated in Fig. 2d. The heavy vectors represent the respective phase directions of the demodulating voltages in the devices I4, I5, 25 and 26 respectively, while the vectors A-A represent the disturbing voltage as applied to the two tubes of each device, alike in each case. By inference from Fig. 2b, two voltages 45 displaced if applied to tube A will produce a substantial positive output while the two voltages 135 displaced applied to tube A will produce only a small opposing output. In consequence a substantial positive voltage will be produced across condenser 2I and the tube I8 will pass a current of the desired magnitude to the transformer 23. Identical net phase displacements will also obtain in the two tubes of device I5 to produce an output voltage in the transformer 24 equal to that in transformer 23 but 90 displaced therefrom. In the devices 25 and 26 the voltages applied to the tubes A are 135 displaced while those applied to tubes A are 45 displaced, the outputs combining to produce a net negative potential across condenser 2| and a zero A. C. output. The phase direction and the magnitude of vector o-e is therefore determined by the devices I4 and I5 acting in concert. By like cooperative action of the devices I4, I5, 25, 26, the neutralizing voltage derived from sourceI I6 is maintained continuously equal in phase and magnitude to the disturbing voltage. This magnitude control of a plurality of sources generating the same frequency but displaced with respect to each other to produce a summation wave of the same frequency but of controlled magnitude and phase, is analogous to that described in Patent 1,688,735, previously mentioned.

A modification of the arrangement of Fig. 2 is shown in Fig. 3. In this ligure some saving in apparatus is accomplished by arranging the devices connected to transformers 23 and 24 to provide complete phase reversals. Thus the rst device will provide a neutralizing potential corresponding with the vectors o-a and o-c of Fig. 2a, while the second produces the Vectors o-b and o-d to complete the 360 rotation. The demodulator stages are identical with those shown in Fig. 2, but their output circuits are balanced and are connected to a second balanced amplifier stage which is incorporated in a bridge circuit including the resistances 3| and 32, while an alternating current neutralizing potential from the source I6 is applied to the midpoint of the bridge through the transformer 30. A maximum potential corresponding to the vector o-a appears in the secondary of transformer 23 when the residual voltage applied to tube A is of identical phase and magnitude with that from the source I6. Under this condition, a zero potential appears across the transformer 24. this reasoning along the lines outlined in connection with Fig. 2, it is seen that the output potential at transformer 23 may vary from one maximum through zero to a maximum in the reverse direction and that the second voltage appearing at transformer 24 may vary likewise but Continuing.

at all times displaced 90 in phase from the output of the rstsystem. When combined in the output circuit 22 a neutralizing voltage obtains from the source l5, which is synchronous with the disturbing voltage and varied in phase in accordance with momentary Variations of said disturbing voltage.

Fig. 4 illustrates still another method of providing a 360 rotation of the neutralizing voltage. In this case the disturbing voltage is applied to a pair of rectifier tubes the grids of which are controlled from the source l5. The grids of these tubes are normally biased strongly negative, so that a current will now only when the positive half cycle of plate voltage is in phase with the positive half cycle of the exciting grid potential. Under these conditions a certain rectified potential appears across the condenser 4i which is then applied to the grid of tube 43. The tubes 43 and 44 comprise a balanced direct coupled amplifier stage, excited from a single grounded source; namely, condenser 4l. IThe output of these two tubes serves to control two amplifier stages and 46 which are arranged to control sources of disturbing voltage displaced zero degrees and 180 degrees, respectively, with respect to the output of generator I6. The output of stages 45 .and 46 is then applied to the neutralizing transformer system via transformers 41 and 48 and amplifier 43.

The operation of the amplifier stage 43 may be further amplified as follows: Assume that a given positive control potential is impressed upon condenser 4I and the grid of tube 43. The plate current of tube 43 will be increased and since this current must iiow through resistance 43a, the cathode of the tube will be made more positive with respect to ground, or conversely, the grids of both tubes will be made negative with respect to their cathodes. With proper adjustment of the resistance 43a and the biasing potential, the grid of tube 44 will bear a potential approximately equal to but opposite in polarity with respect to its cathode from that of tube 43 so that balanced operation is obtained in their output circuits. The negative battery 43h serves to maintain the grids of tubes 43 and 44 at an appropriate operating potential with respect to their cathodes, and the battery 43C performs a like function for the tubes of amplifiers 45 and 46, while at the same time dispensing with coupling condensers. Consequently, the amplifier 43-44 functions as a D. C. amplifier.

This action of the two tubes A and A' of rectiner 40 in conjunction with the condenser 4I, and the similar action of the rectifier 40 and condenser 4i will be made clear by reference to Figure 4a. In this figure the two A. C. potentials, that is the disturbing current and the current from source I6, are illustrated in their respective positions as applied to the four rectifier tubes under four different conditions of phase difference.

For each of the phase conditions the top group applies to tube A of rectifier 40 and the second group to tube A while the lower groups apply to tubes A and A of rectifier 45'. The solid line curves represent the disturbing potential which it may be seen is applied in opposing phase to the two tubes of the respective rectiers. The dotted line curves represent the potential from source i6 which is applied in like phase to the two tubes of each rectiiier but is displaced 90 between the two rectiiers. Inasmuch as the rectiiiers carry a heavy negative bias, only the peaks of rectified waves are effective upon the condensers 40 and 4|'. This bias potential is illustrated in dot-dash lines and the useful rectified current is indicated by the shaded areas.

Considering first the condition when the two potentials are in phase at rectiiier 40A, it is noted that a current in the positive direction is rectiiied to charge the condenser 4I whereby amplifier 43 is energized and as a consequence the amplifier 45 will be rendered operative to pass a neutralizing potential from source i6 in phase with the disturbingr potential, to the transformer 41 and thence to the neutralizing transformer system. No current will pass through the rectiner 40A inasmuch as the plate potential and the regulating grid potentials are 180 out of phase. At the two tubes of rectiner 40 the two applied potentials will be and 270 out of phase respectively, and their summation potentials will be insufficient to overcome the negative bias of the rectifier tubes, consequently no current will be transmitted to the condenser 4l amplilier tubes 43' and 44 remain unchanged and the associated amplifiers 49 and 50 will remain inoperative. Re-stating, a neutralizing current is now being transmitted from source I6, through amplifier 45 to the neutralizing system in phase with the disturbing current applied to rectifier 40A.

Consider now the case where the two potentials are 90 out of phase at rectifier 40A and have respective phase differences at the other rectifier tubes as shown. Under this condition no current is transmitted to condenser 4| but a current of positive polarity is transmitted to condenser 4I'. This charge operating Via amplifier 43 causes neutralizing current from source I6, but 90 displaced to pass via amplifier 49 to the neutralizing transformer system while the remaining amplifiers 45, 45 and 50 remain inoperative.

Under the condition of phase diiference at tubes 40A only the rectifier 40A' is conductive. The condenser 4| is therefore charged negatively, which, operating via amplifier tube 44, paralyzes amplifier 45 but renders amplifier 46 conductive to pass current from source I6 in phase with the control voltage to the neutralizing transformer system. Similarly under the condition of 270 phase difference, the rectifier 40A' is rendered conductive to pass a negative potential to condenser 4| whereby amplifier 49 functions to transmit a neutralizing potential in phase with the control potential to the neutralizing system.

It is now seen that the amplifiers 45, 46, 49 and 50 each functions to transmit a neutralizing potential from the local source I6 having one of four equally displaced phase positions. These four positions correspond to the four vectors of Figure 2a and through their Cooperative functioning a neutralizing potential in phase with the control potential will be continuously sup plied to the neutralizing transformer system.

Figure 5 illustrates another modification of our interference neutralizing system. In this system the principle previously described, whereby the control wire is dispensed with and substantially complete freedom from telegraph distortion is obtained, is applied to the neutralizing transformer scheme of Patent No. 2,159,927 by utilizing a mechanical rectifier and generator of the type shown in Patent No. 1,688,735,

In this system a motor 52 driven synchronously with the disturbing power frequency is coupled to two identical generators, 53, 54, having arma-v tures displaced by 90 and is coupled also to a pair of mechanical rectiers 55 having live segments in 90 relation. Only the slip rings of the two armatures are indicated on the drawings. The residual voltage is selected from the line Wires through high impedances in the manner previously described and is rectified in the manner described in Patent No. 1,688,735. The output from the rectiers is applied to the -eld windings of the two generators via amplifiers 58, 59, arranged to have sufficiently high time constants to provide the requisite selectivity. The amplifiers, of a type which amplifies direct current potentials, have the plate circuits of their output stages embodied in a bridge arrangement, whereby the eld currents for the generators can be reversed. With zero voltages applied to the grids of the first tubes, the condition corresponding to zero interference, the grid bias potentials of the tubes are all so adjusted as to produce a condition of balance in the output stages with a consequent zero current to the field windings of the associated generators. Under these conditions the voltage generated for the purpose of neutralization will be zero. The armatures of the two generators are connected in series and since they may each produce output voltages ranging in magnitude from a maximum in one direction to a maximum in the reverse direction but displaced 90 in phase with respect to each other, a complete 360 phase rotation may be obtained.

An important feature of the interference neutralizing system of Figs. 2 to 5 is their extraordinary stability and freedom from feedback or oscillatory eects. This property is ascribed primarily to the virtual absence of tuning circuits in the systems. Selectivity is obtained almost entirely through the agency of the high time constant resident in the resistance-capacity networks included in the amplifier circuits. A time constant exceeding that obtainable from tuned circuits is preferred for best operation of the system. This may reach as high as 40 seconds. While a tuned circuit I3 is indicated in each of the gures this network has negligible effect upon the selectivity of the system.

We have described and illustrated several embodiments for the purpose of clearly disclosing the invention, but it will be evident to engineers that Various modifications and changes may be made in the circuit arrangements to suit varying conditions.

We claim:

1. In an organization for neutralizing disturbing potentials due to extraneous induction in communication line wires from their proximity to power circuits or other extraneous source, a neutralization system comprising a compensating transformer having windings included in each of said line wires and also a circuit for re- 2. In an organization for neutralizing disturbing potentials due to extraneous induction in line wires derived from power circuits or other extraneous source, a neutralizing system comprising a compensating transformer having windings included in each of said wires, a source of neutralizing potential-independent of said induction in the line wires, an amplifier having its input connected to said source and having its output connected to a winding in said transformer, impedance means connected to saidV lines and selective to the frequency of the induced disturbing interference potentials, and means to apply said selected frequency of said induced interference potentials to the input of said amplifier to thereby control the operation of the neutralizing system.'

3. In an organization for neutralizing disturbing potentials due to extraneous induction in line wires derived from power circuits or other extraneous source, a neutralizing system comprising a compensating transformer having windings included in each of said wires, a source of neutralizing potential independent of said induction in the linelwires, an amplifier having its input connected to said source and havingits output connected to a winding in said transformer, impedance means connected to said lines and selective to the frequency of the induced interference potentials, and a demodulator comprising a pair of thermionic tubes in balanced relation, having their input connected to receive said selected frequency interference potentials, and also connected to said neutralizing potential source, the output of said demodulator tubes being connected to the input of said amplifier, to thereby control the operation of said amplifier.

4. In an organization for neutralizing disturbing potentials due to extraneous induction'in communication line wires derived from power circuits or other extraneous source, a neutralization system comprising a compensation transformer having windings included in each of said line wires, a source of neutralizing potential independent of said induction in the line wires and a control system for continuously regulating the magnitude and phase ofsaid neutralizing potential in accordance with the instantaneous values of the disturbing voltages present in the line wires, comprising means for selecting from certain of said lines disturbing frequency potentials, an amplifier system having its input connected to said potential source and its output connected to a winding of said transformer and means to impress said selected components of the disturbing frequency potentials upon the input of said amplifier system.

5. In an organization defined in claim 4, means to substantially prevent any useful components of the communication currents from affecting the operation of said amplifier.

6. In an organization for neutralizing disturbing potentials due to extraneous induction in communication line wires derived from power circuits or other extraneous source, a neutralization system comprising a compensating transformer having windings included in each of said line wires, a source of neutralizing potential independent of said induction in the linewires and a control system for continuously regulating the magnitude and phase of said neutralizing potential in accordance with the instantaneous values of the disturbing voltages present in the line wires, comprising a circuit arrangement for selecting from said lines disturbing frequency potentials, and a plurality of amplifying and phase control circuits having their outputs connected to a joint conductor terminating in a common winding of said transformer, each such circuit including in combination a pair of demodulator tubes connected in balanced relation with their inputs connected to said source of neutralizing potential and also to said arrangement for selecting disturbing frequency potentials and an amplifier having its input connected to receive the output potential of said pair of demodulator tubes and being also connected to said source of neutralizing potential, the output of said amplifier being connected to said joint conductor.

7. An organization as defined in claim 6, and having a network in the input of the amplifier to provide a high time constant for the control system to thereby render the system non-susceptible to frequencies diifering even slightly from the frequency of the disturbing currents.

8. In a communication system having a plurality of line wires derived from power circuits or other extraneous source subject to disturbing Y potentials due to extraneous induction, a source of potential of neutralizing frequency independent of said induction in the line wires, a neutralizing system comprising a compensating transformer having windings included in each of said line wires, an associated amplifier, said transformer having a winding connected to the input of the amplifier and another winding connected tothe output of theamplier, and a control system having means to select from the line wires disturbing frequency potentials, means to amplify the potential of said source in accordance with said selected disturbing components and means to apply the resultant potentials to said amplier input.

9. A communication system defined in claim 8, having means to orient said resultant potentials to maintain the neutralizing potentials in said transformer in proper phase relation to nullify the disturbing induced potentials.

10. A communication system having a pluralty of line wires derived from power circuits or other extraneous source subject to disturbing potentials due to extraneous induction, a neutralizing system embodying a compensating transformer having windings included in each of said line wires, an amplifier associated with said transformer having its input connected to a winding of said transformer and its output connected to another winding thereof, a control system having means for impressing potentials from a source of neutralizing frequency independent of said extraneous induction in the line wires upon said amplifier input and having an amplitude and phase to cause substantially complete nullification of the disturbing potentials in the line wires.

11. In an organization as set forth in claim 6, an amplifier associated with said compensating transformer having its output connected toV a primary winding of the transformer and its input connected to the opposite terminal of said common winding from the connection of said joint conductor thereto.

12,. In an organization for neutralizing disturbing potentials due to extraneous induction in communication line wires, a neutralization system comprising a compensating transformer having windings included in each of said line wires, a source of neutralizing potential independent of said induction and a control system for continuously regulating the magnitude and phase of said neutralizing potential applied to said compensating transformer in accordance with the values of the disturbing voltages present in the line wires, said control system being operatively connected to the line wires and resonant to the disturbing frequency, a pair of thermionic tubes in balanced relation having their inputs symmetrically connected to receive potentials of said resonant frequency and also from said potential sourceVan amplifier having its input connected to the output of said tubes via a network of high time constant, said amplier being operatively connected to said potential source, said neutralizing system including an amplifier having its output connected to a primary winding of said transformer and its input connected to the output of said amplifier via a winding of said transformer.

13. An interference neutralizing system for line wires in which the neutralizing potential is supplied independently of said line wires and controlled as to characteristics in accordance with the disturbing potential as selected from said line wires.

14. A system for producing a pure alternating current corresponding in phase and amplitude to a like alternating current flowing in a disturbed circuit which comprises an independent generator having the desired frequency, a series of devices for seperately combining components of the generator current with the original current and translating their phase and amplitude diiferences into a series of continuous voltages, and a second series of devices for separately controlling like component currents respectively from said independent source in accordance with said series of continuous voltages and combining them to produce an alternating cur-rent identical with said original current.

15. A communication system subject to variable interference from a primary disturbing source extraneous to said system of constant characteristics, comprising a neutralizing system having means to derive from the said disturbing source, a neutralizing potential corresponding in phase and amplitude to said vari-able interference.

16. In a system for neutralizing inductive interference from an extraneous source i-n signaling conductors requiring a neutralizing potential, means for developing a neutralizing potential from said signaling conductors, comprising synchronously rectifying currents as selected from said conductorsA and means embodying storage circuits for controlling the independent generation of said neutralizing potential,

17. In a system for neutralizing inductive interference from an extraneous source in sig-nal bearing conductors through the application of a neutralizing potential, the method of developing said potential which comprises selecting a control potential intermixed with signals from saidline wires, rectifying said potential in two components displaced in phase and respectively controlling therewith the amplitude of two potentials similarly displaced in phase and combining them to produce said neutralizing potential.

JOSEPH W. MILNOR. WILLIAM D. CANNON ALBERT' BOGGS. 

